Just finished 1st year of PT school and I know how to do this
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We do this under the context of manual therapy and while it does make you feel better and briefly increases your mobility, it doesn't actually fix anything.
Also, most of what these chiros are doing is absolute bullshit. Her cervical spine is not subluxed. That audible pop you hear is not her vertebrae "popping" back into position, it's cavitation of synovium within the capsules of her intervertebral and zygapophyseal joints as the capsule (which is under slight negative pressure) quickly expands. This stimulates the temporary relaxation of some of her tighter deep cervical muscles.
While this is a great precursor to other therapeutic interventions, if this isn't followed by actual treatment, she's just going to feel like shit again in 24 hours.